When further increasing engine efficiency, output power, availability and reliability in the current gas turbine development, this effort is often limited by temperature capacity and lifetime of protective coatings for protection against hot corrosion and oxidation and bonding thermal barrier coating on the hot turbine components. The currently used coatings are so called MCrAlY coatings developed by major gas turbine manufactures. Most gas turbine manufactures use there proprietary coatings since commercially available coatings are much less effective.
The term MCrAlY coatings is widely applied, wherein M means cobalt or nickel or a mixture of cobalt and nickel. These coatings may be used as corrosion resistant overlays or as bond-coats for use with thermal barrier coatings. Since in the first and second stage of a gas turbine metal temperatures may exceed 850° C. two predominant corrosion mechanics have been identified to be accelerated. One is the high temperature oxidation occurring at temperatures above 950° C. and the other mechanism is the so called type-I-hot-corrosion-occurring at approximately 850° C. to 950° C.
During the last ten years the demand for coatings in gas turbines increased, which led to an increase in price of the coatings, which contain comparatively large amounts of rare earth or minor elements which's availability is increasingly tight.
Today's MCrAlY coatings rely very much on yttrium incorporation to have so called pegging and scavenge effects to increase oxidation and corrosion resistance of the coatings.
However, it has been recently found that the yttrium content in MCrAlY may not be optimized. Nijdam T J, Sloof W G. (Acta Materia/ia 2007; 55:5980) reported that yttrium oxide inclusions in the protective aluminum oxide scale on top of the MCrAlY provide fast oxygen diffusion routes and therefore accelerate oxidation of the coating.
Further Smialek J L, Jayne Q T, Schaeffer J C and Murphy W H (Thin Solid Films 1994; 253:285; Smialek J L; Metallurgical Transactions A. Physical Metallurgy and Material Science 1991; 22A:739) state that a high sulfur content of bigger than 10 ppm (parts per million) existed in the current MCrAlY layer shortening coating lifetime.
Typically ceramic coating(s) are used on a hot gas component of a gas turbine, for example turbine blades, combustors, transition ducts, sealing segments, and nozzle guide vanes, providing the components with thermal insulating or sealing functions. For the thermal insulating, the ceramic coatings are called thermal barrier coatings (TBC) which are characterized by a low thermal conductivity and typically consist of zirconia stabilized by yttria generally deposited by plasma spraying and electron beam physical vapor deposition on the substrate. Often a bond-coat is necessary to avoid exceeding residual stresses caused by different thermal expansion of the substrate and the ceramic thermal barrier coating(s). Most often the bond-coat is applied on the surface as a MCrAlY bond-coat.